Radio direction finders (RDF) are well known in the art as tools for finding the direction to a source of electromagnetic radiation. A direction finder may, for example, be a ground-based, an airborne or seaborne system that provides a possibility to locate or monitor various radio signal sources, stationary or movable, by determining the direction of the bearing line to the source. RDFs have many applications which may require the ability to determine the coordinates of radio source over a broad range of broadband frequencies. For instance, wideband RDFs may scan a frequency range of a few MHz to several thousand MHz.
A radio direction finder utilizes an array of antennas (usually two or more) dispersed along the surface of the measurement platform. For ground-based systems, the antennas are typically stationary and the directional information obtained is used to locate the signal source. In aerial or nautical applications, the antennas are carried by a mobile platform, such an aircraft or ship, to receive an electromagnetic signal and process it for obtaining the bearing line from this platform to a source of electromagnetic radiation. In that regard, if the position of the signal source is fixed and known, RDF can be used to determine the platform's position relative to the fixed signal source and, hence, the platform's location. Alternatively, if the platform's position is known, RDF can be used to locate the signal source. For instance, this source can be originated from transmitters used by enemy troops, transmission sources associated with weapons and ordinance, or can be radiation from any type of communications device.
The determination of direction employs amplitude and/or phase comparisons of the signals received by the different antennas from the source of the electromagnetic radiation. Since the antenna's patterns are affected by the platform on which they are mounted a calibration process is required to account for un-calculable effects.
Referring to FIG. 1, the conventional calibration of an airborne radio direction finder can involve the flying of a surveillance aircraft 11 in a horizontal plane in a circular manner so that the aircraft turned 3600 in azimuth. The calibration system includes a calibration transmitter 12 arranged at a ground calibration station in the known location on the ground, and a calibration receiver 13 arranged at the aircraft 11. The calibration transmitter 12 is capable of transmitting electromagnetic signals in a predetermined frequency range while the aircraft moves in the manner that all required azimuth angles φ and depression angles Θ are covered. For instance, the depression angle can be close to 0° when the aircraft is distant from the calibration transmitter 12, while be close to 90° when the aircraft flies directly over the calibration transmitter 12.
Referring to FIG. 2, an exemplary diagram of a conventional calibration electromagnetic signal transmitted by the calibration transmitter is illustrated. For the predetermined frequency range, the calibration signal is sampled over time so that a set of required frequencies f1, f2, . . . , fn is transmitted cyclically to the aircraft from the ground calibration station. As a result of the calibration process, a set of calibration tables is formed establishing a relationship between amplitude and/or phase differences between signals received by the antennas, frequencies f1, f2, . . . , fn, azimuth angles φ1, φ2, . . . , φm and depression angles Θ1, Θ2 . . . , Θk; where n is the number of the measurement frequencies, m and k are the numbers of the selected azimuth and depression angles, respectively, at which the measurements are carried out. The quality of the calibration is dictated by the number n of the frequencies and angle increments (resolution) at which the measurements are carried out, and the purity of the environmental spectrum.
In the conventional calibration technique, each frequency is transmitted/received for a relatively long time period, e.g., 250-500 milliseconds, that results into a rather slow calibration process. Typically, the data, which can be collected during each collection circle of the vehicle, do not exceed the data corresponding to about 10 frequencies. Another drawback of the conventional calibration technique is in the fact that the long transmission time period also increases the probability of being exposed to the interference with the signals originated from external transmitters. Conventionally, if such interference is noticed, the operator usually rejects the collected data, and then resumes another collection of the data with a frequency offset corresponding to the interfered frequency. However, in most cases the operator cannot distinguish between appropriate signals and interfered signals, and therefore cannot eliminate the interference data from the corrupted data in order to use it for generation of a calibration table.
Moreover, in the conventional techniques, synchronization between the ground transmitter and the onboard calibration receiver is based on a detection process. More specifically, in the beginning, the onboard calibration receiver is set to the measurements at the first frequency f1 from the set f1, f2, . . . , fn, as soon as the successful interception of the first signal of the corresponding frequency is performed. Then, the system steps to the next entries of the frequency set. Hence, the conventional synchronization is based on the detection of the first frequency of the set in each retransmission cycle. Any time that this frequency is interfered with the environment electromagnetic signals, it can cause lose of the synchronization. Thus, in order to avoid the interference with the environment electromagnetic signals, the calibration flight, preferably, is carried out at only certain hours of day or at certain distances from the sources of the environment electromagnetic signals. Because of these reasons, the conventional calibration process requires intensive interaction between the ground transmission station and the onboard operators, which by itself increases the interference and demands a skilled ground operator.